Typically, homes with conventional heating and cooling equipment are controlled with a single thermostat which controls the air temperature correctly only in it's own vicinity. Many of these homes will experience temperature variations during both the heating and cooling seasons. In large part, this is due to solar gains, internal loads, outdoor wind, open stairways and many other more subtle factors.
In forced air heating and cooling systems, continuous fan operation can alleviate non-uniform heating and cooling somewhat, but tends to be expensive. Further, in the cooling season, continuous fan operation may cause higher than desirable humidity due to evaporation from a wet evaporator coil while the compressor is off.
In response to these problems and a desire for greater comfort and flexibility, zoned systems have been developed. In these zoned systems, dampers are placed in the ductwork and a thermostat is placed in each zone. Thus, warm or cool air is distributed only where it is needed.
However, control strategies for these zoned systems using a conventional single-firing rate furnace with a single-speed blower and single-speed air conditioner are not necessarily simple. Present day control of zoned systems allow each individual thermostat to turn on the heating/cooling plant and simultaneously open the corresponding zone damper, while closing the dampers to other zones not requiring heating/cooling. With several individual thermostats independently turning the heating/cooling plant on and off, it will cycle significantly more than normal. This has a potential of shortening the life of the equipment. It is, therefore, important to coordinate the thermostat calls and to take advantage of the plant while it is on.
Additionally, when only one zone calls for heat, the entire output of the heating plant (which is capable of heating the entire house) will be supplied to that zone causing larger than normal temperature fluctuations in that zone's air temperature. At the same time, air flow through the furnace is reduced and causes an increase in stack losses and, as a result, the average plenum temperature is increased and the overall furnace efficiency is decreased If the plenum temperature becomes too high because of reduced flow, the furnace burner safety switch will function (high limit operation) and shut off the burner until it has cooled to a preset level. If only one zone continues to call for heat over an extended period of time, the burner may cycle off and on repeatedly from the burner safety switch (called high limit operation). Similarly, in the cooling season, reduced air flow over the evaporator coil decreases the air temperature in the ductwork, possibly leading to ice formation on the cooling coil.
One solution to these problems has been proposed in U.S. patent application, Ser. No. 07/451,705, filed on Dec. 18, 1989, and assigned to the assignee of the present invention. In that patent application, a thermostat in the zone of greatest thermal demand is used to control both a duct damper for that zone and the heating/cooling plant. The remaining thermostats in the zones of lesser thermal demand have their cycle rate and, hence, their duty cycles modified so that they control their respective duct dampers in synchronization with the "on" period of the thermostat of the zone of greatest demand.
Although this approach has been successful, the present invention discloses another more sophisticated and improved solution to this problem.